Retro-reflectors are optical devices which have the property of reflecting incident light back in the direction of the light source. These devices are often designated less exactly as reflex reflectors or reflectors. In principle there are two systems that are retro-reflecting, namely the triple mirror and the ball reflex elements (BRE) which are also known under the name of "cats' eyes".
Retro-reflectors manufactured from glass are becoming increasingly prevalent by reason of their special properties and especially by reason of their long working life and ability to retain high reflection values.
Retro-reflectors are used, for example, to increase safety on streets at night, along the edges of roadways or on the roadways themselves. The effectiveness of these retro-reflectors, in addition to their optical construction, is largely dependent on regular maintenance and care. In consequence of their exposed position on streets, for example, as ground marking nails, or in the immediate vicinity of streets, for example, as guidepost reflectors, all retro-reflectors are subject to extreme fouling. So that they can function properly, therefore, it is necessary to clean them regularly. The necessity for continuous cleaning can lead to progressive as well as considerable losses in reflection.
Thus, for example, reflex reflectors that are made of plastic, lose after about 25 cleanings up to 90% of their original reflecting capacity. Considerably better results are obtained with retro-reflectors of glass. The latter, after the same cleanings, lose only about 10% of their original reflecting capacity, and maintain these high reflecting values for years. Retro-reflectors of glass, therefore, have significant advantages over those made of plastic. For certain reflector applications glass retro-reflectors are used exclusively.
The production of shaped bodies of glass adapted for use as retro-reflectors presents, however, exceedingly higher manufacturing demands than the manufacture of plastic retro-reflectors.